Two types of source may have that kind of characteristic:
sources having internal modulation, e.g. a semiconductor laser, in which power of emission is modulated by varying the electrical current injected into the laser; or
sources having external modulation, such as a laser that emits continuously and that is associated with an electrically controllable amplitude modulator. For example, the modulator may be an interferometer of the Mach-Zehnder type, or of the electro-absorption type.
FIG. 1 is a block diagram of an externally modulated light source. It comprises: a semiconductor laser 1; a Mach-Zehnder type modulator 2; and an analog adder 3. The laser 1 emits a light beam of constant power P.sub.0, which is applied to one input of the modulator 2. The modulator has an electrical control input on which it receives a voltage V.sub.c supplied by the output from the adder 3. An optical output of the modulator 2 provides an amplitude-modulated light beam of variable power P. The adder 3 sums the DC bias voltage V.sub.b with a modulation voltage V.sub.m representing a string of binary data.
FIG. 2 shows the characteristic of that source, i.e. it is a plot of the output power P as a function of the control voltage V.sub.c. The plot P(V.sub.c) of a perfect modulator of the Mach-Zehnder type is a sinewave of period 2V.sub.p, where the value of the half period V.sub.p is inherent to the structure of the modulator. To achieve modulation, the characteristic is made use of over a portion of the sinewave that has a slope of constant sign, e.g. the region AB, with this being done by placing the operating point on a point of inflection C of the sinewave. This characteristic is liable to drift, particularly under the influence of temperature, with the characteristic moving parallel to the abscissa axis. Consequently, the portion ACB of the sinewave moves relative to the bias voltage which is taken as consisting a fixed point.
To transmit a digital signal, e.g. including two or three discrete levels, it is not absolutely essential for modulation to be linear. It is therefore possible to make use of the entire region AB so as to achieve maximum modulation depth. The output power P then varies between 0 and P'.sub.0 which is the maximum power delivered by the modulator when it has the power P.sub.0 applied to its input.
A method of obtaining such modulation consists in applying a control voltage V.sub.c to the control input of the modulator, where the voltage V.sub.c has an amplitude of .+-.1/2V.sub.p centered on a value V.sub.0 corresponding to the point of inflection C which is the center of symmetry of the region AB. Under such circumstances, it is necessary to apply a bias voltage V.sub.b =V.sub.0 to one of the inputs of the adder 3, and to apply a modulation voltage V.sub.m of amplitude .+-.1/2V.sub.p and centered on zero voltage to the other input.
In the example shown in FIG. 2, the voltage V.sub.0 constituting the optimum bias voltage has some arbitrary positive value. In practice, the voltage V.sub.0 is not constant. It drifts, in particular as a function of the temperature of the modulator 2. The entire characteristic P(V.sub.c) moves parallel to the abscissa axis V.sub.c. If the bias voltage V.sub.b is fixed, then it cannot coincide with the optimum voltage V.sub.0 when it drifts. Consequently, the emitted light signal is deformed, either in the vicinity of point A or else in the vicinity of point B of the plot of P(V.sub.c).
FIG. 3 shows this deformation phenomenon where solid lines are used to show the output power P(t) as a function of time t, for a binary signal taking the values 101 when the drift in the characteristic has caused the operating point to move towards point A of the characteristic. The output signal then has a low level of power P.sub.1, that is non-zero, and a high level of power that passes twice through the value P'.sub.0, but that includes a relative minimum between those two maxima.
The transmitted signal is deformed since the high level is no longer of constant power. In addition, the power difference P'.sub.0 -P.sub.1 is smaller than when the operating point remains accurately in coincidence with the center of symmetry C of the characteristic. In FIG. 3, the waveform of the output signal under such circumstances is shown in dashed lines.
This drift in the characteristic causes the eye diagram to close up somewhat, thereby increasing the transmission error rate. That is why it is necessary to servo-control the bias voltage V.sub.b to variations in the voltage V.sub.0 corresponding to the center of symmetry.
Two methods are known of servo-controlling the bias voltage V.sub.b on the optimum value V.sub.0.
A first servo-control method is described in the article "Novel automatic bias voltage control for travelling-wave electrode optical modulators", Electronics Letters, Vol. 27, May 23, 1991 by Kataoka et al. In that method, a second light flux passes through the modulator in the opposite direction to the light flux being modulated by the data to be transmitted. This second light flux is also modulated, so by observing its modulation it is possible to servo-control the bias voltage.
The main drawback of that method is the use of an additional light source and of two polarization-maintaining couplers which are difficult to connect to the modulator. As a result there are problems with the reliability of the apparatus. In addition, such a device poses problems of bulkiness and of insertion losses in the two optical couplers; it is also expensive.
A second servo-control method is described in the article "Automatic bias control circuit for Mach-Zehnder modulator", IEICE, Spring National Convention Record, B-976, 1990, by Kuwata et al. A laser emitter emits light flux of constant power which is applied to a Mach-Zehnder modulator. A signal representing binary data at high frequency is applied to a high frequency modulator that also receives a sinewave signal at low frequency and small amplitude. The high frequency signal modulated by the low frequency signal is applied to the Mach-Zehnder modulator. A fraction of the light signal output from the modulator is taken off by an optical coupler and is then applied to a detection diode which is followed by a phase comparator. The phase comparator compares the phase of the detected signal with that of the low frequency signal. A lowpass filter filters the signal delivered by the phase comparator. The filtered signal represents the voltage difference that exists between the bias voltage applied to the Mach-Zehnder modulator and the voltage at which modulation is optimal, i.e. the DC voltage V.sub.0. This voltage difference can be used to change the bias voltage.
Thus, in that second method, the peak-to-peak amplitude of the modulation signal is modulated about its mean value by a sinewave signal of small amplitude and low frequency. Such modulation of the modulation signal induces low frequency sinewave modulation of the mean light power. The phase position of said sinewave modulation relative to the sinewave signal applied to the high frequency modulator has the same sign as V.sub.b -V.sub.0. By determining this phase difference it is possible to servo-control V.sub.b on V.sub.0.
Unfortunately, implementing that method requires a Mach-Zehnder modulator to be used with its gain being modulated by the low frequency signal. Such low frequency modulation behaves like noise relative to the transmitted signal, thereby degrading the signal-to-noise ratio. Furthermore, it becomes very difficult to make a high frequency electronic modulator for amplitude modulating the high frequency signal with a low frequency signal when the rate at which the digital values are transmitted is very high.